1. Field of the Invention
The present invention relates to an automatic frequency control circuit for a digital transmitter-receiver unit in a digital communication system, and more particularly to an automatic frequency control circuit capable of inhibiting a distortion phenomenon caused by a variation in frequency involved in a time division duplex (TDD) communication system.
2. Description of the Prior Art
The presently available digital cordless telephone systems adopt, as a transfer technique, a TTD system or time division multiple access (TDMA)-TDD system in which signal transmitting and receiving operations are carried out in a ping-pong manner using a single frequency.
FIG. 1 is a block diagram illustrating a digital transmitter-receiver circuit which adopts a conventional continuous automatic frequency control method. Such a transmitter-receiver circuit is disclosed in U.S. Pat. No. 5,309,429 invented by Kuno Fukuda, assigned to Sony Corporation and issued on May 3, 1994. This transmitter-receiver circuit includes an automatic frequency control circuit which serves to control a local oscillator included in the receiver part of the transmitter-receiver circuit. When an input signal exhibiting a high frequency offset is received at the receiver part of the transmitter-receiver circuit, the automatic frequency control circuit feeds back a voltage signal corresponding to the frequency offset, thereby varying the frequency of an output signal from the local oscillator. The automatic frequency control circuit includes a frequency discriminator 72 and a lowpass filter 73. An output signal from the frequency discriminator 72 is accumulated as a reference voltage in a memory internally included in the lowpass filter 73. Based on the reference voltage, the automatic frequency control circuit controls the frequency of a carrier signal Si sent to a modulator 84 included in the transmitter part of the transmitter-receiver circuit and the frequency of a signal Sj sent to the local oscillator of the receiver part. In this way, the automatic frequency control circuit carries out an automatic frequency control function.
FIG. 2 is a circuit diagram illustrating a detailed configuration of a part of the digital transmitter-receiver circuit shown in FIG. 1. Since a frequency drift may occur while the transmitter-receiver circuit receives incoming calls, the automatic frequency control circuit included in the transmitter-receiver circuit performs an automatic frequency control for each incoming call.
In a transmitting interval, a switch 733 of the low-pass filter 73 is switched on by a control signal S.sub.37 output from a system control unit 31. Accordingly, the output voltage V.sub.72 of the frequency discriminator 72 becomes smooth through capacitors 732 and 734. As a result, the output voltage of the frequency discriminator 72 is averaged and then accumulated in the system control unit 31 as a reference voltage.
In a receiving interval, the same automatic frequency control operation, as in the transmitting interval, is carried out. That is, the switch 733 of the low-pass filter 73 is switched on by the control signal S.sub.37 output from the system control unit 31 in the transmitting interval. Accordingly, the output voltage V.sub.72 of the frequency discriminator 72 becomes smooth through the capacitors 732 and 734. As a result, the output voltage of the frequency discriminator 72 is averaged and then transmitted to the system control unit 31.
Then, the system control unit 31 compares the output from the low-pass filter 73 received therein in the receiving interval with the reference voltage accumulated in the transmitting interval, thereby detecting a difference between the compared voltages. Based on the voltage difference, the system control unit 31 adjusts the oscillating frequency of the voltage-controlling local oscillator in such a manner that the voltage difference becomes zero.
However, the above-mentioned conventional automatic frequency control system may have the following problems where it is applied to a frequency division multiple access-time division duplex (FDMA-TDD) communication system.
First, the FDMA-TDD communication system carries out transmitting and receiving operations at different time intervals, respectively. Although the local oscillator generates a normal frequency having no frequency deviation (.DELTA.f=0) from a central frequency in a transmitting interval of the communication system, a variation in frequency may occur within a tolerance from the central frequency (namely, a 50% range from .+-.10 KHz defined in accordance with the CT-2 CAI Standard) in a receiving interval of the communication system. In this case, the communication system performs an automatic frequency control, thereby tracing the output signal from a demodulation unit in the form shown in FIG. 3. In FIG. 3, the phantom line indicates an ideal reference signal which is constant in amplitude depending on time. However, a variation in amplitude occurs in a practical case, as shown by the solid line of FIG. 3. In the receiver circuit of a digital cordless telephone, it is essentially necessary to generate a reference signal for shaping the digital waveform of digital data radio-received in the receiver circuit in the form of a signal suited for a base-band signal processing unit upon recovering the digital data. As a result, the reference signal may vary continuously in amplitude due to a continuous small variation in the received frequency. In this case, a frequency distortion may occur. Furthermore, a degradation in signal to noise (S/N) ratio also occurs. As a result, the sensitivity of the practical system to incoming data may be degraded.
Second, where the conventional continuous automatic frequency circuit is applied to the signaling hierarchy of a second generation digital cordless telephone (CT-2), it is difficult to appropriately carry out a frequency control for each signaling layer. For this reason, it is impossible to achieve an efficient automatic frequency control. This results in a degradation in the performance of the whole system. Of course, it is possible to satisfy the demand of each signaling layer using the continuous automatic frequency circuit. In this case, however, the configuration of the circuit is considerably complicated.